Occurrence of bacterial black shoot disease of European pear in Yamagata Prefecture

Black lesions on shoots of European pear trees observed in an orchard in Yamagata Prefecture in May 2007 were suspected to be caused by a bacterial pathogen. The surface of the colonies isolated on a high sucrose medium did not have the crater morphology that is characteristic of E. amylovora bvs. 1–3, and a specific DNA fragment was amplified from the isolates in the PCR using the EprpoD primer set. The partial sequences of the 16S rRNA gene placed the isolates in the genus Erwinia. The isolates differed serologically from E. amylovora biovars and E. pyrifoliae in an Ouchterlony double-diffusion test although their bacterial properties suggested that they are closely related to E. amylovora biovars and E. pyrifoliae. In a DNA–DNA hybridization test, the relatedness between the isolates and E. amylovora biovars or E. pyrifoliae did not exceed 70% level, indicating that they are independent species. Thus, the isolates belongs to the genus Erwnia but are not E. amylovra or E. pyrifoliae. After succulent pear shoots were injected with bacterial suspensions (10⁹, 10⁸, 10⁷ and 10⁶ cfu/ml) of the isolates, lesions formed with 10⁹ and 10⁸ cfu/ml, but the disease incidence with 10⁸ cfu/ml was much lower than with E. amylovora and E. pyrifoliae. Virulence of the present isolates is thus thought to be very weak. On the basis of these results, we consider that this is a new shoot disease of European pear. In the 2007 season, all affected trees were pulled out after harvest. No symptoms have been observed in field surveys since the fruitlet season in 2007.